1. Field of the Invention
This invention relates to a vibration wave driven motor in which electrical energy is converted into vibration energy by an electro-mechanical energy conversion element and a mechanical output is obtained from the vibration energy through friction.
2. Related Background Art
Generally, a vibration wave driven motor is comprised of a vibration member having an electro-mechanical energy conversion element such as a piezo-electric element secured to an elastic member, and a movable member brought into pressure contact with the elastic member. An AC voltage is applied to the two driving phases of the piezo-electric element to excite a travelling wave in the elastic member and the elastic member and the movable member are moved relative to each other of the vibration energy by the travelling wave through the frictional force with the movable member, and the relative movement is used as the output of the motor. Accordingly, the elastic member is formed of a material of small vibration loss (internal loss) for the purpose of improving the efficiency of the motor, and as the material of the elastic member, use is made of a material of very high Q, e.g. Q=100-300 so that vibration may be readily generated by small energy.
Now, in such a vibration wave driven motor, the movable member is in a state in which it is pressed against the elastic member to take out the output of the motor. Therefore, the elastic member is subjected to a vertical force (a force in the direction of pressure contact) and a frictional force (a moment by a frictional force) during the driving of the motor, and in some cases, vibrations other than the driving wave are also excited due to such forces.
These vibrations are chiefly the natural vibrations of the vibration member (though the vibration member is weakly compounded with the movable member through a frictional force), and provide noise when the frequencies of these vibrations are within an audible range. Also, even if the frequencies of these vibrations themselves are not within the audible range, there will be created the difference thereof from the vibration frequency of the driving mode and a component harmonic wave by the non-linearity of the vibration of the vibration member in a state in which it is combined with the movable member, and if these are within the audible range, they will pose a problem as noise.
I have made motors of various types by way of trial and have studied the vibration mode when the problem of noise arises.
As a result, I have found that there are chiefly two types of noise, one of which is a component harmonic wave of the driving mode created by forced vibration and the other is self-excited vibration created by a frictional force.
It has further been found that the cause of creation of the latter self-excited vibration is divided chiefly into two cases, i.e., a case where it is determined by the dimensions of the movable member and the vibration member, and the dynamic rigidity, friction constant, etc. of the contact portion, and a case where it is caused in a mode one order lower than the driving wave (the out-of-plane mode), irrespective of the dimensions. Further examination of the situation in which the self-excited vibration occurs in the vibration mode one order lower than the driving wave (the out-of-plane mode) has found that the self-excited vibration occurs when the contact between the movable member and the elastic member is not kept uniform.
FIG. 2 of the accompanying drawings shows a typical example of the vibration spectrum at such time. The vibration member used is a circular ring-shaped one. The ordinate represents the output from a sensor layer provided on the vibration member (a sensor layer which produces a voltage conforming to the vibration of the vibration member by the piezo-electric effect, and the abscissa represents frequency.
"f" represents the characteristic in the driving vibration mode (the out-of-plane flexure 7-th-order mode of the circular ring), and "g" represents the characteristic in the vibration mode which has occurred as the self-excited vibration by a frictional force, i.e., the characteristic of the vibration in the mode one order lower than the characteristic "f", i.e., the out-of-plane flexure 6th-order mode. A characteristic "h" is the difference between the characteristic "f" and the characteristic "g", and vibration of about 8 KHz of this characteristic causes noise. Thus, I have confirmed empirically that if there is irregularity of contact (irregularity of pressure) between the elastic member and the movable member, the vibration mode one order lower than the driving wave (i.e., vibration having a number of waves less by one than the number of waves of the driving wave) occurs and any unnecessary vibration mode other than this vibration mode, for example, the vibration mode two orders lower than the vibration mode of the driving wave, does not occur. That is, it has been found that when there is irregularity of contact (irregularity of pressure), the vibration which causes noise is limited to the vibration of the mode one order lower than the driving mode.
Also, examination of the noise when the irregularity of contact (the irregularity of pressure) is small has found that the noise vibration mode changes depending on the structure, materials, friction constants, driving order numbers, etc. of the movable member and elastic member, but in the case of the driving of seven waves, the driving wave is limited to one of three, four, five and six waves or a combination thereof.
FIG. 9 of the accompanying drawings shows a typical example of the noise when the irregularity of pressure is null.
"f" represents the characteristic of the driving vibration mode, "k" represents the characteristic of the 3rd-order vibration mode, and "j" represents the characteristic of the 4th-order vibration mode.
It has been found from what has been described that where there is adopted structure in which no irregularity of pressure occurs, for a case where the driving wave comprises seven waves, a mode free of noise can be realized if a countermeasure for noise is applied to the 3rd-order, 4th-order, 5th-order and 6th-order modes.
On the other hand, it has already been proposed in U.S. application Ser. No. 480,201 filed on Feb. 14, 1990 to provide a portion of non-uniform rigidity in the elastic member or the like as a countermeasure for noise.
The principle of driving of the vibration wave driven motor is that by an AC electric field being applied to the piezo-electric element, two standing waves positionally deviating from each other by .pi./2, i.e., a standing wave of sin mode and a standing wave of cos mode, are excited in the elastic member and a travelling wave is formed by the combination of the two standing waves, and if a difference occurs between the natural frequencies of the two standing waves, a travelling wave will no longer be formed.
From this, it follows that to prevent the generation of a travelling wave of the unnecessary order number which causes noise, a difference in natural frequency may be provided between two standing waves in that order number.
As a method of providing a difference between the natural frequencies of standing waves, for example in a case where the elastic member is of an annular shape and a plurality of slits are formed at equal pitches in the upper surface thereof along the circumferential direction thereof, the depth of the slit at a location corresponding, for example, to the node of one standing wave but to the antinode in the other standing wave is made greater than the depth of the other slits, whereby it becomes possible to provide a difference between the natural frequencies.